Hot melt compositions produced by irradiating ethylene copolymers in the presence of wax

ABSTRACT

Hot melt compositions are produced by preparing a concentrate of wax and copolymer, irradiating the concentrate to produce an irradiated concentrate and then diluting the irradiated concentrate with additional quantities of wax to obtain the hot melt composition. The improved three step method permits the incorporation of copolymers having higher molecular weights in the hot melt composition; it was also found that the copolymers are more readily dissolved in the wax. The copolymers used are the ethylene-vinyl acylate and ethylene-alkyl acrylate copolymers.

United States Patent [191 Domine et a1, 7

[73] Assignee: Union Carbide Corporation, New

York, NY.

[22] Filed: July 20, 1972 [21] Appl. No.1 273,542

52 0.5.0. 204/159.14, 260/285 AV s1 Int.Cl B0lj l/10,B0lj 1/12 58 Fieldof Search 204/159.14; 260/285 AV [56] References Cited UNITED STATESPATENTS 3,436,325 4/1969 Aufhauser 204/1592 June 25, 1974 9/1970 Potts..204/159.14 5/1973 Tubbs ..204/159.14

[ 5 7] ABSTRACT Hot melt compositions are produced by preparingaconcentrate of wax and copolymer, irradiating the concentrate to producean irradiated concentrate and then diluting the irradiated concentratewith additional quantities of wax to obtain the hot melt composition.The improved three step method permits the incorporation of copolymershaving higher molecular weights in the hot melt composition; it was alsofound that the copolymers are more readily dissolved in the wax. Thecopolymers used are the ethylene-vinyl acylate and ethylene-alkylacrylate copolymers.

v6 Claims, No Drawings HOT MELT COMPOSITIONS PRODUCED BY IRRADIATINGETHYLENE COPOLYMERS IN THE PRESENCE OF WAX BACKGROUND OF THE INVENTIONThe addition of ethylene-vinyl acylate copolymers or ethylene-alkylacrylate copolymers to wax to produce wax blends having improvedproperties is well known and has been practiced for many years, as shownby U.S. Pat. No. 2,877,196. Also known is the addition to wax ofcopolymers of ethylene and vinyl acetate that have been irradiated withhigh energy ionizing radiation, prior to the blending, as shown by US.Pat. No. 3,530,084, or the irradiation of a mixture of polyethylene andwax, as shown in US. Pat. No. 3,436,325. The presence of the polymer inthe hot melt composition upgrades the performance of the wax for use inhot melt coating and adhesive compositions.

The improvement obtained by the presence of the polymers in the blend isknown to increase as the molecular weight of the polymer increases, thatis, as the melt index decreases. Thus, the most desirable practice wouldbe the use of polymers or copolymers having very high molecular weights;it also being known that the copolymers are more efficacious. However,the dissolution of very high molecular weight copolymers in wax withoutthermal or therrnomechanical degradation of the polymer or wax isdifficult. The high temperatures and severe mixing conditions requiredwith the very high molecular weight copolymers are known to result indegradation of the copolymer and the finished product does not containthe high molecular weight copolymer but instead contains a thermallydegraded, lower molecular weight copolymer. The actual and relativeamount of degradation which occurs increases with the initial molecularweight of the copolymer, that is, a low molecular weight copolymer woulddegrade lesson both an absolute and proportional basis than a very highmolecular weight copolymer. As molecular weight of the copolymer isincreased, a point of diminishing returns is reached where increasingthe molecular weight of the copolymer does not increase the molecularweight of the copolymer in the hot melt measurably due to the increasedamount of degradation.

In most commercial hot melt mixing operations wherein a copolymer havinga melt index of 1.5 dgm./min. or higher is dissolved in wax, the mixingoperation is generally carried out at temperatures of 100C. or higher inorder that the solution be completed in a commercially acceptable periodof time. For these relatively high melt index, relatively low molecularweight copolymers these conditions are satisfactory and there is noindication of significant copolymer degradation. However, the moredesired hot melt compositions are those containing a relatively highmolecular weight copolymer having a melt index below 1.5 dgm./min.,preferably below 1 dgm./min. In order to prepare these compositionstemperatures above 150C. are usually required, often 200C. or higher, tocompletely dissolve the polymers in the wax. Even at these highertemperatures, it has been necessary to carry out the hot melt mixing formuch longer periods of time, and the combination of these harsherconditions has generally resulted in significant and severe copolymerdegradation.

DESCRIPTION OF THE INVENTION We have now found an improved methodwhereby a wax-copolymer hot melt coating and adhesive composition can bereadily produced in which the copolymer in the coating and adhesive hotmelt composition has a melt index below about 1.5 dgm./min. Thesecompositions can be readily and rapidly produced without thedisadvantages previously encountered.

In the process of this invention a relatively low molecular weight,relatively high melt index ethylene-vinyl acylate copolymer orethylene-alkyl acrylate copolymer is initially compounded with wax at alow temperature, generally at a temperature from about 60C. to C. thoughlower or higher temperatures can be used. In most instances, but notnecessarily, less than the total amount of wax desired in the finalwaxcopolymer hot melt composition is used; that is, a major amount offrom about 50 per cent or more of the relatively low molecular weightcopolymer is compounded with a minor amount of wax in this initial step.The product of this initial step is referred to in its specification asthe concentrate. This initial step can be carried out in conventionalpolymer processing equipment (Banbury mixer, hot melt extruder, rollmill, Brabender mill, etc.) and, though not preferred, there need not becomplete solution of this copolymer in the wax, Since the copolymer usedto prepare the concentrate has a relatively high melt index and is onewhich dissolves readily in the wax there is no problem in the productionof the concentrate and there is no evidence of extensive thermal ormechanical degradation of the copolymer. The starting copolymers used inthis initial step have a relatively high melt index of from about 1.5dgm./min. to about dgm./min., preferably from about 1.5 dgm./min. toabout 30 dgm./min. The process however is not restricted to the use ofsuch copolymers and copolymers having lower or higher melt indices canbe used; however, the maximum advantages of this invention are observedwith the use of such copolymers initially.

In the second step of the process of this invention the concentrate isexposed to radiation, the product of this second step is referred to inthis specification as the irradiated concentrate. The irradiationresults in an increase in molecular weight of the copolymer as evidencedby a decrease in the melt index, without any noticeable effect. on thewax present in the irradiated composition. The irradiation can becarried out on the molten composition or on the composition in solidform; any temperature from about 0C. or lower up to the melting point ofthe composition, or higher, can be used provided that the temperature isnot so high as to cause degradation of the wax or the copolymer. Thepreferred temperature is from about 20C. to about 40C.

The irradiation is carried out with a sufiicient dose to lower theinitial melt index of the copolymer in the composition. This dose willvary depending on the initial melt index of the copolymer and the finalmelt index desired, the temperature at which the irradiation is beingconducted, the physical form of the concentrate that is being irradiatedand other factors known to affect the rate of reaction. The dose willnormally range from 0.01 to 5 megareps of absorbed dose.

At the completion of the irradiation the copolymer in the irradiatedconcentrate has a lower melt index than the initial copolymer used toprepare the concentrate had prior to irradiation. This melt index cannow be as low as about 0.01 dgmjmin. The particular value of the .meltindex of the copolymer in the irradiated concentrate is primarily amatter of choice and the reaction is controlled by the operator by knownmeans to obtain the desired relatively low melt index. Thus, one canstart with a concentrate containing an ethylene-vinyl acetate copolymerhaving a relatively high melt index of about 10 dgm./min. and produce anirradiated concentrate wherein the copolymer has a relatively low meltindex of less than 0.2 dgm./min.; alternatively, one can irradiate thesame copolymer to the extent that the copolymer in the irradiatedconcentrate has a relatively low melt index of 1.2 dgm./min. Or, one canstart with a concentrate containing an ethylene-vinyl acetate co polymerhaving a relatively high melt index of about dgm./min. and product anirradiated concentrate wherein the copolymer has a relatively low meltindex of about 0.85 dgm./min. In every instance, however, there is adecrease in melt index and an increase in molecular weight of thecopolymer. The melt index of the copolymer in the irradiated concentratecan be as low as 0.1 percent of the value of the melt index of theinitial copolymer used to prepare the concentrate that is subsequentlyirradiated. The preferred irradiated con- 1 centrates are those in whichthe copolymer has a relatively low melt index below 1.0 dgm./min. In thepast it has been difficult to produce hot melt wax-copolymercompositions by the known methods containing copolymers having such lowmelt index values. The copolymer after irradiation is free of noticeablegel particles which would present problems in the subsequent step andultimate use of the hot melt composition.

The source and type of radiation is not critical and any form ofionizing radiation can be used. Gamma rays, X-rays, beta radiation,protons, neutrons, and alpha particles may all be used. These are wellknown, as are the means for obtaining them, and do not require anyfurther description to enable one skilled in the art to understand whatis meant.

The copolymers are the known copolymers of ethylene and a vinyl acylateor an alkyl acrylate containing 5 percent or more, usually 15 to 40percent by weight, of polymerized vinyl acylate or alkyl acrylate. Thevinyl acylate can have from 2 to 4 carbon atoms in the acylate group;the alkyl acrylate can have from 1 to 4 carbon atoms in the alkyl group;also included in the term alkyl acrylate are the free acrylyl acids andthe alkyl methacrylates. Illustrative of suitable copolymers one canmention the copolymers of ethylene with vinyl acetate, vinyl propionate,vinyl butyrate, methyl acrylate, ethyl acrylate, propyl acrylate,isopropyl acrylate, butyl acrylate, methyl methacrylate, ethylmethacrylate, isobutyl acrylate, butyl methacrylate, acrylic acid,methacrylic acid, and the like.

In the third step of the process of this invention the irradiatedconcentrate is dissolved in additional quantities of wax to obtain thedesired concentration of copolymer in the hot melt coating and adhesivecomposition. This dissolution is carried out by conventional means. lthas been found, however, that the time required for this dissolution inthe additional wax was much shorter than the time that would be requiredto dissolve a copolymer having essentially the same low melt index ashas the copolymer in the irradiated concentrate by the previouscommercial methods. Not

only was the time unexpectedly less, but the temperature at which thedissolution was carried out can be lower. The temperature of thisdissolution can be from about 110C. to about 150C. even though thecopolymer in the irradiated concentrate has a melt index below 1.5dgm./min. This discovery is of great commercial significance since itdecreases the cost and time for producing the final hot melt coating andadhesive compositions and also eliminates the problem of thermal andthennomechanical degradation of the components thereof. The process alsomakes possible the ready preparation of hot melt coating and adhesivecompositions containing high molecular weight (low melt index)copolymers that in many instances were not heretofore commerciallyproducable. The product of this third step is the desired hot meltcoating or adhesive composition and is referred to in this specificationby that designation or by the term hot melt composition.

The hot melt compositions can also contain the other additivesconventionally present, such as antioxidants, inhibitors, fillers,pigments and colorants, polymer modifiers, modifying polymers, syntheticresins and waxes, surface reactive agents all of which are known in theart.

The following examples further serve to illustrate the invention. Theprocedures used in evaluating the compounds were: ASTM D l238-65T forthe melt index and ASTM D l505-63T for the density.

EXAMPLE 1 A series of three concentrates was produced by initiallycompounding mixtures of wax and ethylene-vinyl acetate (EVA) copolymerin a Banbury mill at 70C. for 10 minutes. The copolymer used has aninitial relatively high melt index of 20 dgm./min. and a vinyl acetatecontent of 28 weight percent.

The three concentrates were irradiated in air at about 20C. with highenergy electrons by exposure to the radiation from a two MEV van deGraaff electron accelerator. The irradiated copolymer in the irradiatedconcentrates now had a low melt index calculated to be about 1.2dgm./min. This value is estimated on the basisthat the melt viscositiesof these irradiated concentrates were similar to the melt viscosity ofthe control at the same concentration and temperature; the

' control was produced usinga copolymer having a melt index of 1.2dgm./min. The dose was adjusted so that a hot melt blend of 40 weightpercent irradiated copolymer and 60 weight percent wax would have aboutthe same melt viscosity as the comparative control blend hereinafterdescribed. When all other factors are equal, e.g., concentration,temperature, etc., melt viscosity can be used as a measure of molecularweight. The three irradiated concentrates were diluted with wax to yieldhot melt coating and adhesive compositions containing 40 weight percentcopolymer. This dilution was carried out in a standard commercial-typelaboratory mixer at C. to C.

The comparative control blend was prepared by mixing 40 parts ofethylene-vinyl acetate copolymer having an initial melt index of 1.2dgm./min. and a vinyl acetate content of 28 weight percent with 60 partsof wax in a commercial-type laboratory mixer.

The details and results are summarized in the following table: I

Run A B C Control Concentrate. pts. by wt.

EVA (Mi-=20 dgmJmin.) 80 85 90 Wax 20 l l0 0 Irradiated concentrateDosage. megareps 1.5 1.3 0.9 0 Hot melt Composition, pts.

Irradiated concentrate A 50 0 0 0 Irradiated concentrate B 0 47.1 0 0Irradiated concentrate C 0 0 44.4 0

EVA (Ml=l.2 dgmJmin.) 40

Wax 50.0 52.9 55.6 60

Mixing time, min. 1 130 145 260 Viscosity at 121.1" C., cps. 42,30039,900 41,800 42,800

The data show that the total mixing time for the preparation of Run Awas only 44 percent of the mixing time required for the preparation ofthe control; for the preparation of Run B only 50 percent and for thepreparation of Run C only 56 percent. These reductions in time are ofgreat commercial significance and were completely unexpected.

EXAMPLE 2 The concentrate from Run B of Example 1 was irradiated with adose of 0.78 megarep to produce an irradiated concentrate D. Theirradiated copolymer in this irradiated concentrate now had a melt indexcalculated to be about 5 dgm./min. This irradiated concentrate was usedto produce a hot melt composition by mixing gel particles were observed,and the weight averagev molecular weight of the copolymer afterirradiation; also determined was the melt index of the irradiatedcomposition. The presence of gels was determined by dissolving thesample'in hot toluene, casting a film on a glass plate, evaporating thetoluene, and examining the plate visually forevidence of insoluble gels.At doslrradiated concentrate A B C D Control EVA 90 85 80 70 100 Wax l015 20 0 Dose, megareps 1.35 1.80 1.95 2.15 1.12 Melt index, dgm/min.0.169 0.185 0.59 4.6 0.059 Mol.wt. X 10 576 535 477 352 reas mv'ss wishRun D Control Hot melt composition Irradiated concentrate D, pts. 47.1 0EVA (M|=5 dgm./min.), pts. 40 Wax, pts. 52.9 60 Mixing time, min. 85 125Viscosity at 121 1C., cps. 16,700 17,600

EXAMPLE 3 A series of concentrates was prepared each concentratecontaining a major amount of an ethylene-vinyl acetate copolymer havinga vinyl acetate content of 28 weight percent and a melt index of 6.6dgm./min., and

a minor amount of wax. The concentrates were produced by compounding atabout 70C. The concentrates, and the starting copolymer itself, wereirradiated in a two MEV van de Graaff electron accelerator and agelevels higher than those set forth, one would detect gels.

The irradiated samples described above were used to produce hot meltadhesive compositions containing 20 percent and 40 percent copolymercontents by two procedures.

Procedure I The irradiated sample and wax were hot melt blended at about120C. in a laboratory mixer. It was found that the time required forcomplete solution was less for the irradiated concentrates than the timerequired for the irradiated control. The saving in mixing time was ashigh as 48.7 percent as shown by the data reported by the designationMixing time reduction, percent. The low temperature brittlenessproperties, T of the hot melt blends containing 40 percent copolymerwere determined and it was found that the hot melt blends produced usingthe irradiated concentrates were tougher than the hot melt blendproduced from the irradiated control.

Procedure 11 The irradiated sample and wax were dissolved in hot benzeneat about C. The solution was then vacuum dried to remove the solvent.This procedure avoids therrnomechanical degradation of the copolymer andhence the blends have higher melt viscosities than the correspondingblends produced by Procedure A.

In all instances, the melt viscosities of the blends produced using theirradiated concentrates were higher than the melt viscosities of thecorresponding irradiated control.

We claim:

1. In the method for producing hot melt compositions PROCEDURE 1Irradiated concentrate A B C D Cont. EVA in final blend, 71 20 40 20 4020 40 20 40 20 40 Mixing time. min. 245 320 210 285 195 240 210 195 260380 Mixing time reduction, 7: 5.8 15.6 19.2 25 25 36.8 19.2 48.7 Meltviscosity X 10*" 2.86 198 3.16 189 198 2.63 212 2.50 148 T;,.,. "C 29-32 33 35 -28 At l21.lC. of the solvent free EVA-wax compositionPROCEDURE 11 A 1 B C D Cont. 4O 2O 40 2O 4O 2O 40 20 40 EXAMPLE 4 of waxand an ethylene-vinyl acylate copolymer or an I A concentrate wasproduced by hot compounding 10 parts of wax with 90 parts of an ethyleneethyl acrylate copolymer having an ethyl acrylate content of 18 weightpercent and a melt index of 20 dgm./min. The concentrate was exposed tothe ionizing radiation from a two MEV van de Graaff electron acceleratorto impart a total dose of 2.05 megareps. A portion of this irradiatedconcentrate was hot melt blended in an additional quantity of wax atabout 120C. in a laboratory mixer to produce a hot melt compositioncontaining 40 weight percent of the copolymer. The time required forthis latter dissolution step was 135 minutes. The hot blend compositionhad a melt viscosity at l21.lC. of 18,400 centipoises. The melt index ofthe copolymer in the irradiated concentrate after irradiation was about6 dgmJmin.

For comparative purposes 60 parts of wax and 40 parts of an ethyleneacrylate copolymer having an ethyl acrylate content of 18 weight percentand a melt index of 6 dgm./min. were hot melt blended in the samemanner. The time required to obtain complete dissolution was 195minutes, which is a 44.5 percent longer period of time than thatrequired to dissolve the irradiated concentrate above. The meltviscosity of the control hot melt blend was 22,000 centipoises.

The hot melt compositions of the examples, which were produced in accordwith the process of this invention, were equal or superior, in general,in heat seal performance and ageing properties than were theirrespective comparative control compositions.

ethylene-alkyl acrylate copolymer wherein the vinyl acylate or the alkylacrylate content is from 5 to 40 weight percent, the improvement ofproducing said hot melt compositions by the steps of (a) producing aconcentrate of said wax and up to a major amount of said copolymerhaving an initial melt index of from 1.5 dgm./min. to dgm./min., (b)irradiating said concentrate with high energy ionizing radiation of anabsorbed dose of from 0.01 to 5 megareps to increase the molecularweight of said copolymer and thereby produce an irradiated concentrate,and (c) dissolving said irradiated concentrate in an additional quantityof wax to produce a hot melt composition of wax and said copolymer, saidcopolymer in said hot melt composition having a lower melt index thansaid copolymer in said concentrate.

2. The improved method claimed in claim 1, wherein the vinyl acylate insaid copolymer has from 2 to 4 carbon atoms in the acylate group.

3. The improved method claimed in claim 1, wherein the vinyl acylate insaid copolymer is vinyl acetate.

4. The improved method claimed in claim 1, wherein the alkyl acrylate insaid copolymer has from 1 to 4 carbon atoms in the alkyl group.

5. The improved method claimed in claim 1, wherein the alkyl acrylate insaid copolymer is ethyl acrylate.

6. The improved method claimed in claim 1, wherein said copolymer usedto produce the said concentrate has an initial melt index of from about1.5 dgm./min. to 100 dgm./min. and after irradiation the copolymer inthe irradiated concentrate can have a melt index as low as O. 1 percentof the value of said initial melt index. =l=

2. The improved method claimed in claim 1, wherein the vinyl acylate insaid copolymer has from 2 to 4 carbon atoms in the acylate group.
 3. Theimproved method claimed in claim 1, wherein the vinyl acylate in saidcopolymer is vinyl acetate.
 4. The improved method claimed in claim 1,wherein the alkyl acrylate in said copolymer has from 1 to 4 carbonatoms in the alkyl group.
 5. The improved method claimed in claim 1,wherein the alkyl acrylate in said copolymer is ethyl acrylate.
 6. Theimproved method claimed in claim 1, wherein said copolymer used toproduce the said concentrate has an initial melt index of from about 1.5dgm./min. to 100 dgm./min. and after irradiation the copolymer in theirradiated concentrate can have a melt index as low as 0.1 percent ofthe value of said initial melt index.